Thank you.My own personnel experience, not slating the product or any others like it!
As a Truck owner, and have been for over 10 years (mostly Volvo FH's and an R500 V8), I can honestly say if it's MPG you want to save, and trust me I want to save them in my trucks, where 0.1 MPG is around £2k pa, nothing will beat sitting back, chilling out and lifting your right foot 1cm, using the CC at every given moment and doing around 5-10mph less than the posted speed. Other owner drivers I've spoken to who have had these types of devices fitted and when comparing MPG I always win, I'm also never late.
My thoughts on these devices, from my own research is that they try to trick the ECU into something that isn't actually happening, may be telling the ECU the throttle is at 90% when your giving it full beans or messing with the input of the MAF. Either way, would a company manufacturing and selling these have the same R&D as VW? That ECU in your vehicle knows the exact fuel air ratio every 1/10th of a second, how far forwards and advanced the timing is amongst a myriad of other readings, of only some are available to the OBD2 port every 1 second, which is what this plugs into.
Now, if you want more MPG/PS/BHP then your going to need to remap that ECU, and this of course invalidates any warranty, as this adjusts things like the fuel air mixture, throttle input loads and most importantly, the turbo/s settings.
As I said this is my experience, I have tried a couple of these, in a Golf I used a Bluefin plug and go, and some others I care not to remember, and yes they did make a difference, but never as much of a difference as changing my driving habits, and that is free. If you want to throw cash at it do an IAM or ROSPA course.
If you made it this far congratulations, and also apologies for the length.
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